In VL it happens only if you choose tmpfs as your /tmp . Then your /tmp directory will be a virtual one, without using your hd but only RAM (as if it was /proc) . Problem with that is when I wanted to write a dvd, k3b wanted to use /tmp as a temporary dir. and I ran out of ram. I had to configure k3b to use /home/kidd/tmp as its temporary dir.

After saying that, I don't know of a reliable way to clean /tmp on each boot. maybe playing with your /etc/rc.d